1. Field of the Invention:
The present invention relates to a variable energy ion beam source for use with a mass spectrometer. More specifically, it relates to an analytical apparatus utilizing a gas chromatograph and a mass spectrometer in which the mass spectrum is swept by varying the energy of the ion beam source.
2. Discussion of the Prior Art:
Both gas chromatographs and mass spectrometers have been used as analytical tools. It has long been recognized that a powerful analytical tool could be obtained by the coupling of these two instruments. However, gas chromatographs generally operate at atmospheric pressure while mass spectrometers operate at a greatly reduced pressure. To account for this, some interfacing means must be provided to reduce the pressure of the sample gas leaving the gas chromatograph before it is introduced into the mass spectrometer. Furthermore, since gas chromatographs operate by sweeping a small amount of sample gas through a column, using a high volume of a carrier gas, some means must be found to enrich the concentration of the sample gas relative to the carrier gas before the gas mixture reaches a mass spectrometer. Failure to do this will reduce the sensitivity of the mass spectrometer.
A gas chromatograph separates the various components of a sample gas so that the composition of the gas leaving the chromatograph varies with time. Because of the continually changing composition of the gas stream reaching the mass spectrometer, any mass spectrometer which is designed for use in conjunction with a gas chromatograph must be one capable of sweeping rapidly across the mass spectrum, so that the changing composition of the output of the gas chromatograph is reflected. For mass spectrometers of the magnetic sector design, mass sweeping can be accomplished by either varying the magnetic field or by varying the energy of the ion beam. Varying the magnetic field, however, is a comparatively slow process, so there is an advantage to sweeping the energy of the ion beam.
In the past, magnetic mass spectrometers were massive structures in which the entire ion beam, including the ion beam source, were contained in the magnetic field. The amount of metal acquired to produce such a magnetic field is uneconomical, so in recent years, the size of the magnet has been reduced to the point where only a small segment of the ion beam path actually passes between the poles of the magnet. At least for those situations in which the ion beam source is located outside of the analyzing magnetic poles, no satisfactory solution to the production of an ion beam by varying the potential of the ion beam source has been found. Such sources can be produced when the energy of the beam is swept over a small range of energies, but in the situation where it is necessary to sweep the energy of the beam over a wide range of energies to encompass a large portion of the mass spectrum, ion beam focus over the entire range has not been achieved. Focus can be achieved at one energy, but as the energy changes, the focus of the ion beam changes, and eventually the ion beam is extinguished.
There are a number of additional problems that arise in an analytical system combining a gas chromatograph with a mass spectrometer utilizing a variable energy ion source. For one thing, since some means must be utilized to decrease the pressure in the interface between the gas chromatograph and the mass spectrometer, from about atmospheric pressure in the chromatograph to about 0.001 Torr in the ion source, the pressure in the interface must pass through a region which is ideally suited for gas discharge. When combined with the high energy of the ion beam source, this region of reduced pressure produces a gas discharge in the connecting line. For obvious reasons, this is unacceptable.